Intensive residential development of land along the northwest shore of Topaz Lake has raised concern regarding the adequacy of the water supply and possible effects of septic-tank effluents on the quality of local ground water and the lake. Existing data on water levels, well construction, and water quality were compiled, and a limited program of water-level measurement and water-quality sampling was conducted during 1976-77 to provide documentation of current hydrologic conditions against which effects of future development may be measured.
Water-level measurements show the presence of small independent aquifers near the periphery of the alluvial fan covering the study area and a main alluvial aquifer in which water levels fluctuate in direct response to annual changes in lake stage.
The sampled ground water can be grouped into four chemical types--three based on the relative proportions of sulfate and bicarbonate and one based on an increased proportion of chloride plus nitrate.
Frequency distributions of the observed occurrences of nitrate (as NO3), phosphate (as PO4), and chloride in ground water indicate thresholds for background concentrations of these constituents at 0 to 3, 0 to 0.35, and 0 to 6 milligrams per liter, respectively. Above-background concentrations of nitrate and phosphate in ground water may be attributable, in part, to natural sources of these constituents; however, septic-tank effluents have increased the concentrations of nitrate, chloride, and perhaps phosphate in at least two areas of the current development. Sampling for dissolved organic carbon and coliform bacteria in ground water detected no contamination by these potential indicators of pollution.
